Lithium 18650 voltage limits?

[SamTexas]

I'm starting to play with a bunch of Sony Energytec US18650S cells.

Two questions on the upper and lower voltage limits:
1) Max 4.30V: Is this the voltage "UNDER CHARGED" or "AT REST"? I am charging them at 0.5C or less.
2) Min 2.50V: Again, is this "UNDER DIScharged" or "AT REST"? I am discharging at 5 to 10C.

And what is the optimum charging rate (if any) for 18650 cells? 0.5C, 1.0C, 2.0C or what?

Thanks,
Sam

 

[grindz145]

4.3 is max charge voltage. I wouldn't bring them that high. At rest it would be more like 4.15.

optimum charge rate is as slow as possible. 1.0C is not advisable, I would go .5C max. They'll prolly take 1.0C but its detrimental to the cells.

 

[amberwolf]

Here's a document from Sony about their Lithium batteries, which has quite a bit of data in there.
It's a PDF, zipped up so the board would take it.

I can't find my Sony spec sheets, but on a couple of Sanyo 18650 spec sheets, I found that they recommend pretty different charging setups:

The UR18560SAX specifies a charge voltage of 4.20V, at a max of 820mA (for a 1250mAh cell, making it about 0.75C), giving it a charging time of 3 hours. This results in a final nominal charged voltage of 3.7V (once the surface charge burns off in the first moments of use).

The UR18650F specifies the same charge voltage and end-result voltage, but allows a max of 2100mA for a 2100mAh cell, meaning a 1C charge. Specifies a charge time of 2.5 hours, but one of those has to be wrong, as it won't take that long to charge it AFAICS, at that rate.

The latter cell also gives some charts, which show that you don't get much extra usable Ah out of it past the 4V charge point anyway, so you could lengthen their lifespan by not going above about that. Also shows that past about 3.3-3.5V, there isnt' much usable energy down there, either, so I wouldn't recommend discharge past that point--it'll extend life if you don't.

At 2.5V, they're totally used up capacity-wise, and drop like a rock off that cliff starting around 3.5V depending on the rate you're discharging them at. If your cell-level LVC doesn't work fast enough to stop discharge at that point, then the cell(s) that keep dropping are likely to be damaged.

I expect the Sony batteries are just about the same.

 

[SamTexas]

Thank you amberwolf for the detailed reply. It's very helpful, especially the PDF file.